The invention relates to friction clutches, and more particularly to improvements in friction clutches which are automatically adjustable to compensate for wear upon their pressure plates and/or friction linings.
A friction clutch of the character normally employed in motor vehicles to transmit torque between the combustion engine and the driven wheels, particularly between the output element of the engine and the input element of a variable-speed transmission, normally comprises a pressure plate which is axially movably coupled to a cover or housing. The cover is affixed to a counterpressure plate (such as a flywheel) which receives torque from the engine, and a spring (such as a diaphragm spring) is installed between the cover and the pressure plate to urge the pressure plate against one friction lining on a clutch disc or clutch plate serving to transmit torque to the input element of the transmission. Another friction lining bears against the counterpressure plate when the clutch is engaged. Disengagement of the clutch involves axial movement of the pressure plate axially and away from the counterpressure plate, and the friction linings. The friction linings are subject to wear as a result of repeated engagement and disengagement of the friction clutch.
Presently known proposals to compensate for wear upon the friction linings of a friction clutch are disclosed, for example, in German Pat. No. 29 20 932 and in published German patent application Serial No. 35 18 781. Thus, an axially movable adjusting device is installed between the diaphragm spring and the pressure plate. The actual axial adjustment is or can be effected by wedges which bear against the pressure plate and can move the adjusting device toward the diaphragm spring in response to angular displacement of the adjusting device relative to the pressure plate. It was also proposed to change the positions of the wedges for the purpose of moving the adjusting device relative to the pressure plate.
The means for ascertaining the extent of wear upon the friction linings in conventional friction clutches of the above outlined character comprises a plurality of sensors which operate between the pressure plate and the counterpressure plate (normally a flywheel) or between the pressure plate and the cover. When the clutch is engaged, the sensors are displaced in dependency on the extent of wear upon the friction linings, and the extent of movement of the pressure plate away from the counterpressure plate in response to disengagement of the clutch is altered in dependency on the extent of adjustment of the sensors. The operation of conventional automatic wear compensating systems is based on the assumption that, when the friction clutch is new, the disengagement involves a movement of the pressure plate (away from the counterpressure plate) which exactly matches the extent of movement of the diaphragm spring. When the friction linings undergo a certain amount of wear, the pressure plate moves toward the counterpressure plate (flywheel) to an extent which is dependent on the extent of wear. Therefore, in the absence of any undertakings to the contrary, the diaphragm spring is then required to cover a greater distance from the cover toward the counterpressure plate in order to bias the pressure plate against the adjacent friction lining, i.e., to clamp the friction linings of the clutch disc between the pressure plate and the counterpressure plate. In other words, the distance which is covered by the diaphragm spring then exceeds the distance covered by the pressure plate. This results in appropriate axial displacement of the adjusting device in a direction toward the diaphragm spring.
A drawback of the conventional proposals is that the adjustment does not always correspond to the extent of wear upon the friction linings. The reason is that the extent of movement of the diaphragm spring in response to engagement or disengagement of the friction clutch fluctuates within a certain range, even if the extent of movement to disengage or engage the friction clutch is constant. The bearing which forms part of the disengaging means and serves to displace the customary prongs of the diaphragm spring during disengagement of the friction clutch is particularly likely to perform movements which depart from the expected or prescribed movements within a rather wide range. Furthermore, when the clutch disengaging mechanism (e.g., a mechanical disengaging system) is adjusted, its actual adjustment is likely to depart from an optimal adjustment, for example, because the clearance between the disengaging system and the diaphragm spring is too small and/or because the initially selected stress between the disengaging system and the diaphragm spring is too pronounced. This results in the establishment of operating points which depart from the optimal operating point of the friction clutch.
If a friction clutch is equipped with an automatically adjustable disengaging system e.g., a hydraulic system which employs a master cylinder and a slave cylinder, the aforediscussed means which is to compensate for wear upon the friction linings cannot be operated at all because the extent of movement to disengage the friction clutch is the same during the entire useful life of the clutch. Thus, no adjustment will take place as long as the movement of the diaphragm spring in the region of the adjusting device during disengagement of the friction clutch is less than or at most matches the extent of movement of the pressure plate. On the other hand, if the extent of movement of the diaphragm spring exceeds the extent of movement of the pressure plate, the adjusting device is actuated and carries out an adjustment during each disengagement of the friction clutch irrespective of the extent of wear (or any wear) upon the friction linings so that the friction clutch is totally misadjusted after a relatively small number of disengagements.
An additional problem which arises in conventional self-adjusting friction clutches (i.e., in clutches wherein the relative positions of the pressure plate and the diaphragm spring are to be automatically changed as the wear upon the friction linings progresses) is that the pressure plate is likely to become disengaged from the diaphragm spring in the axial direction of the clutch in response to characteristic resonant vibrations. This results in activation of the adjusting device which induces a total misadjustment of the friction clutch and renders it useless for the intended purpose.